In product development every new feature or functionality being built needs to work with every feature already built. Even though not all new features affect all previous features but all combinations of new features with old features need to be considered to find the old features that do get affected. The new feature may affect the previous features through either code or product logic. This checking of every new feature with all the existing ones is what slows down both product and engineering. The effect of the 51st feature needs to be checked on the first 50 features. Because of this the number of features you have built act as a cumulative tax on the development of new features. And there comes a point where the tax is so high that almost all of the time is spent paying this tax and only a small amount of time gets used up in developing new features. The effect of this tax becomes worse when the companies get big and parts of product logic and code are divided among multiple people so that one person doesn't understand all the old parts or when a new person joins. So even more time is spent in first trying to understand all the old features that exist and then trying to figure out which ones will be affected. This is also why the older and larger the company the slower development. Slow speed of development is inherent to age and size of a company. This problem is so bad that there are products that try to help you learn the existing functionality in a company so you can build new functionality (getonboardai.com)